A. Field of the Invention
This invention relates to a robot and method for automatically bending orthodontic archwires, retainers, or other orthodontic or medical devices into a particular shape.
B. Description of Related Art
In orthodontics, a patient suffering from a malocclusion is treated by affixing brackets to the surface of the teeth and installing an archwire in the slots of the brackets. The archwire and brackets are designed to generated a customized force system that applies forces to teeth, by which individual teeth are moved relative to surrounding anatomical structures into a desired occlusion. There are two approaches to designing an appropriate force system for a patient. One is based on a flat archwire and customized brackets, e.g., Andreiko et al., U.S. Pat. No. 5,447,432. The other is based on off-the shelf brackets and designing a customized archwire that has complex bends designed to move or rotate the teeth in the desired direction. Traditionally, the latter approach has required manual bending of the archwire by the orthodontist.
Machines for bending orthodontic archwires have been proposed in the prior art. Andreiko et al. describes an apparatus that takes a straight archwire and imparts a simple planar arcuate curvature to the wire. The wire is customized in the sense that the shape of the arc is designed for a particular patient, but the wire bending apparatus described in Andreiko et al. is limited to a customized bracket approach to orthodontics. In particular, the Andreiko et al. wire bending apparatus cannot produce any complex and twists bends in the wire, e.g., bends requiring a combination of translation and rotational motion.
The patent to Orthuber et al., U.S. Pat. No. 4,656,860 describes a bending robot for bending archwires. A robot as described in the ""860 patent was developed by the predecessor of the assignee of the present invention and used experimentally for several years, but never widely commercialized. The robot consisted of two characteristic design features: a bending cone that could move forwards and backwards to bend the wire, and a rotating cone that could twist the wire. As such, it could only apply torque or bends over the two main axes of a cross section of a rectangular shaped wire. Since the portion of the wire extending beyond the cone is free and unconstrained, the robot had no control as to the effective deformation of the wire. Additionally, a series of three twists and two bends were typically required by a robot in accordance with the ""860 patent to shape an archwire so that it would fit in the slots of two adjacent brackets. This series of twists and bends required as much as 5 mm of wire length between adjacent brackets. This length of wire is greater than that available for closely spaced teeth, such as the lower front teeth. To avoid this situation, the robot bent a twisted portion of the wire, which provoked uncontrolled rotational motion in the wire.
The design of the ""860 patent also has other shortcomings: it provides no means for measuring forces imparted by the wire since one end of the wire is free and the wire is gripped immediately below the bending point. The robot had no effective feedback mechanism for detecting how the wire in fact was bent after a particular bending or twisting operation was performed. As the free end of the wire is not constrained or held in any manner, there is no ready way to heat the wire as it is being bent in order to fix the shape of the bend in a wire made from a shape memory material. Consequently, shape memory alloy wires made with the ""860 patent were subject to a separate heating treatment in a separate thermal device.
The present invention presents a substantial improvement to the robot of the ""860 patent. The invention also provides for much greater flexibility in the design and manufacture of archwires than that disclosed by the Andreiko et al. patent. In particular, the present invention enables the manufacture of custom, highly accurate orthodontic archwires. Such wires are ideally suited to an archwire-based orthodontic treatment regime based on standard, off-the-shelf brackets. The invention is also readily adaptable to bending other medical devices, including implants such bone fixation plates, prostheses, orthotic devices, and even surgical tools.
In a first aspect, a bending apparatus or machine is provided for bending an orthodontic appliance, such as a retainer or archwire, into a desired configuration. While the orthodontic device is described as being an archwire in the illustrated embodiment, other types of medical devices are contemplated as the type of article capable of being bent by the robot. Examples of such medical devices are prostheses, orthotic devices, implants, fixation plates, spectacle frames, and surgical devices such as a reamer for root canals.
The bending apparatus or machine may take the form of a robot mounted to a base or table support surface. A first gripper tool is provided. This tool can either be fixed with respect to the base or may be incorporated into a moveable arm. The first gripping tool has a first gripping structure for holding the archwire or other medical device. The bending apparatus includes a moveable arm having a proximal portion mounted to the base a distance away from the first gripper tool and a free distal end portion. The moveable arm is constructed such that the free distal portion of the moveable arm is capable of independent movement relative to the first gripper tool along at least one translation axis and about at least one rotation axis. In an illustrated embodiment, the moveable arm has a set of joints which allows the distal end of the arm to move in 6 degrees of freedomxe2x80x943 orthogonal translational axes and 3 orthogonal rotational axes. However, depending on the nature of the medical device and the required bends to form in the device, a lesser number of degrees of freedom may be appropriate, reducing the cost and complexity of the bending apparatus.
A second gripping tool is mounted to the distal portion of the moveable arm. The second gripping tool has a gripping structure for gripping the archwire. Thus, the archwire is gripped by the first and second gripping tools, with the second, moveable gripping tool capable of motion relative to the first gripping tool along at least one translational axis and at least one rotational axis.
The robot further includes a control system operative of the moveable arm and the first and second gripping tools so as to cause the first and second gripping tools to grip the archwire while the gripping tools are separated from each other and to cause the second gripping tool to move about at least one of the rotational axis and translation axis to thereby bend the archwire a desired amount. Preferably, the control system reads an input file containing information as to the shape of the archwire (or location of bending points along the wire) and responsively operates the moveable arm and first and second gripping tools to form a series of bends and/or twists in the archwire.
The nature of the bends in the archwire will be dictated by the orthodontic prescription and the type of force system that the orthodontist has chosen for the patient. Complex bends involving a combination of bends and twists are possible with the robot. For such complex bends, it has been found that a six-axis robot, in which the second gripping tool is capable of movement relative to the first gripping tool about three translation axes and three rotation axes, is a preferred embodiment.
Orthodontic archwires and other medical devices may have elastic properties such that when a certain amount of force is applied to the workpiece, it returns to its original configuration at least to some degree. What this means is that when a certain bend is formed in the wire, say a 10 degree bend, the wire may take a shape of an 8 degree bend due to this elastic property. Hence, some overbending of the archwire may be needed to account for this elastic deformation. Solutions for overbending wire are provided. One method is a force-based approach. In this approach, the robot comprises a force sensor system for detecting forces generated by the wire after the wire has been bent by the first and second gripping tools. Depending on the direction and magnitude of the detected forces, additional bends are formed in the wire. The proper bend in the wire is deemed to exist when the wire, at its designed shape, exhibits zero or substantially zero forces.
An alternative approach to overbending is based on deformation. In this approach, the wire is bent, the wire is released from the moveable gripping tool and a measurement is made of the wire""s shape, the wire is bent some more (assuming more bending is required), the wire is released again, and the process continues until the resulting configuration is the one specified by the input file. In this embodiment, a camera or other optical technique can be used to measure the shape of the wire. Alternatively, force sensors can be used to determine the actual bend in the wire (by moving the moveable gripper holding the wire to the position where no forces are measured), and a measurement is taken to indicate what additional bends, if any, are needed to result in the desired configuration.
It is further contemplated that a database of overbending information can be acquired as the robot bends wires. This database of overbending information can be used by artificial intelligence programs to derive a relationship between overbending and desired bends, for a particular archwire material. It may be possible to overbend wires in a single step, that is without requiring a lot of intermediate bending steps, based on this database of information, or based on a derived relationship between overbending and resulting wire shape.
In another aspect, the robot includes a heating system to apply heat to the archwire or other workpiece while it is in the bent condition and/or during bending. A current-based resistive heating system and heated grippers are used in the illustrated embodiment. This system allows shape memory alloys to be bent by the robot and the acquired bends retained in the wire material. Other heating systems are possible depending on the nature of the device being bent.
In another aspect, the robot is part of an archwire manufacturing system including a magazine containing a plurality of straight archwires. The magazine holds the archwires such that they are spaced from each other so as to enable the robot to grip an individual one of the archwires. Several different magazine designs are proposed. After the robot has formed the archwire, the archwire is placed at a finish location. A conveyor system carries the finished archwire from the finish location to a labeling and packaging station. The wires are individually labeled and packaged. Alternatively, pairs of wires could be labeled as corresponding to a single patient and packaged together.
In still another aspect, a gripping tool for a bending robot is provided. The gripping tool includes a pair of opposing gripping fingers moveable between open and closed positions, and a force system coupled to the gripping fingers for detecting forces"" imparted by a workpiece such as an archwire or other medical device after a bend has been placed in the workpiece. As noted above, the force system can be used to measure resulting forces after a certain bend has been placed in the wire, and the measurements used to indicate additional bending steps to yield the required configuration taking into account the need for overbending.
In still another aspect of the invention, a method is provided for bending an orthodontic archwire in a bending robot. The method includes the steps of
a) gripping the archwire with a first gripping tool such that a portion of the archwire projects beyond the first gripping tool;
b) gripping the portion of the archwire extending beyond the first gripping tool with a moveable gripping tool;
c) releasing the gripping of the archwire by the first gripping tool;
d) moving the moveable gripping tool while gripping the archwire so as to draw the archwire through the first gripping tool a predetermined amount;
e) the first gripping tool again gripping said archwire after the step of moving is performed, and
f) moving the moveable gripping tool relative to the first gripping tool so as to place a bend in the archwire having a desired configuration.
In the above method, the moveable gripping tool and first gripping tool can cooperate to place a series of bends in the archwire. It has been found that the movement called for by step f) should be performed such that a constant distance, equal to the length of archwire pulled through the fixed gripping tool in step d) is maintained between the fixed gripping tool and the moveable gripping tool. This distance should be maintained in order to avoid applying tension or compression to the wire. Since the moveable gripping tool is moving potential in three dimensions during the bending, the distance that needs to be maintained is measured along the length of the archwire. The same principle holds true for bending other types of devices.
These and still other aspects of the invention will be more apparent in view of the following detailed description of a presently preferred embodiment.